Recent years have witnessed impressive developments in light-emitting elements in which a light-emitting layer is formed using a nitride semiconductor (hereafter, nitride), and wherein light emission takes place through recombination of positive holes and electrons in the light-emitting layer when current is injected from outside. Some of the light emitted by the light-emitting element excites a phosphor. The light generated by the phosphor and the light from the light-emitting element are mixed to yield white light that is used as a light source. The use of such light sources in illumination devices has received much attention. However, no light sources that meet high-efficiency requirements have been achieved thus far. In particular, two main factors bring efficiency down in the process whereby white light is generated using a phosphor.
Firstly, part of the energy is lost (Stokes' loss) upon wavelength conversion. Specifically, some excitation light emitted by the light-emitting element and absorbed by the phosphor is converted to light, which is outputted to the exterior, and that has a wavelength of lower energy than the energy of the light emitted by the light-emitting element. This loss and efficiency drop occur in proportion to the difference between the energies of the excitation light from the light-emitting element and the light emitted by the phosphor.
Secondly, efficiency drops also on account of non-emissive recombination in the phosphor (drop in internal quantum efficiency in the phosphor). Specifically, crystal defects in the phosphor function as non-emissive recombination centers. Thus, some of the carriers generated in the phosphor by the excitation light do not contribute to emission but remain trapped in the crystal defects, lowering as a result the emission efficiency of the phosphor.
Obtaining white light using a phosphor through the above-described two stages entails a significant drop in efficiency that precludes achieving high-efficiency light-emitting elements. The above explanation is set out in cited Patent document 1, which was proposed earlier by the present applicant. In addition, the use of the above-mentioned phosphors is accompanied, in sulfide-, silicate- and halosilicate phosphors, by hydrolysis (hydration reactions) caused by moisture, and by fast deterioration on account of excitation light, for instance UV light. Such phosphors are therefore problematic in terms of low reliability and short life. Other problems when using phosphors are poor color rendering and poor hue. Specifically, realizing white light emission with high color rendering involves at present a tradeoff between color rendering and luminous efficiency, given the weak emission from red phosphors. At the same time, no high-efficiency phosphors have yet been achieved in UV-light-emitting semiconductors through excitation of RGB three-color phosphors.
In the current state of the art, therefore, there is no alternative to using RGB three-color chips to realize highly reliable white LEDs that boast high color rendering. The difficulties involved in designing optical systems that are free of color variation, and the difficulties involved, in terms of cost, in applying the above technologies to ordinary illumination, constitute added problems.
To tackle the above technical problems, the present applicant proposes a compound semiconductor light-emitting element that enables multicolor emission, such as white emission, in one chip, using the above-described columnar crystalline structures, but no phosphor. Specifically, crystal growth nuclei are grown at a temperature lower than the ordinary growth temperature of the columnar crystalline structures and then the temperature is raised over time up to the ordinary growth temperature, to confer thereby variability to the nuclei. Thereafter, the columnar crystalline structures are grown as usual, to impart thereby variability to the thickness and/or composition of the light-emitting layer, and cause the columnar crystalline structures to emit at different wavelengths. The growth of the above columnar crystalline structures is described in, for instance, Patent document 2.
The procedure set forth in Patent document 1 is excellent in terms of realizing a solid-state light source that enables multicolor emission in a simple manner, and hence at low cost, since it relies on a single substrate and a single growth step. Since multicolor emission is made possible by uneven growth, however, the above technique is problematic in terms of low precision upon coordination of the emission colors of solid-state sources into a desired hue when the technique is used, for instance, in illumination applications.    Patent document 1: JP 2007-123398 A    Patent document 2: JP 2005-228936 A